A considerable amount of effort has been expended developing methods of manufacturing semiconductor integrated circuit chip packages. For example, the assignee of the subject invention has developed an automated tape bonding process for fabricating such chips. This process is described in U.S. Pat. No. 4,701,781, issued Oct. 20, 1987, to Sankhagowit, the disclosure of which is incorporated herein by reference. The development of another similar type of chip package is described in U.S. Pat. No. 4,796,080, issued Jan. 3, 1989 to Phy, and incorporated herein by reference.
As described in U.S. Pat. No. 4,701,781, during fabrication of chip packages utilizing the tape bonding technique, an integrated circuit die is bonded to a copper tape. The copper tape includes a plurality of conductive leads which are electrically connected to the integrated circuit die. The die is then encapsulated using a plastic material in a molding step. Simultaneously, a carrier is encapsulated peripherally around and spaced from the die package. The resultant package 10 is shown in FIG. 1. During the encapsulation process a gate 12 is formed between the die 13 and the carrier 14.
In order to keep the leads 15 co-planar and properly aligned during the fabrication steps, conductive dambars or tie bars 16a and 16b are initially formed on the copper tape 17 and located to be near the inner die 13 and the outer carrier 14 respectively. The dambars 16 also act as a stop to prevent the plastic encapsulant from flowing out between the leads and past the dambars. During the encapsulation step, some of the flowing plastic will, however, be trapped between the die and the dambar 16a (shown at 18 in FIG. 1) and between the carrier and the outer dambar 16b (shown at 20 in FIG. 1).
Once the encapsulation step is completed, the conductive copper dambars 16a and 16b and the trapped plastic flash 18, 20 must be removed. In the past, this removal was done mechanically with a punch. This approach is satisfactory as long as the spacing between the leads does not get too small. However, as the number of leads increases and the spacing between the leads decreases, it becomes quite difficult to mechanically remove the dambars. Moreover, when the dambars are placed very close to the encapsulant, the problems with mechanical removal become more acute. In the prior art, laser machining systems have supplanted mechanical machining approaches in certain applications where high precision is required, such as in trimming resistors. However, no attempts have been made to utilize a laser machining system to remove the dambars as well as plastic flash trapped by the dambars from an integrated circuit package.
In order to develop such a system, a number of difficulties must be addressed and solved that were not encountered in other laser machining applications. For example, it was recognized that dambar removal and plastic deflashing operations required two different intensity laser beams. More specifically, the copper dambars must be cut with a relatively high intensity laser beam. In contrast, since plastic has a lower melting point, a lower intensity beam could be used. In fact, a lower intensity beam should be used for a number of reasons. For example, the removal of trapped plastic flash from between the leads requires that the beam be directed quite close to the integrated circuit package. If a high powered beam were used, the die could be damaged. In addition, a higher power beam interacting with the plastic encapsulant could form carbon deposits which are conductive and could lead to shorted leads and package failure.
Accordingly, to carry out the subject invention a means is needed to generate laser beams of different power. This need could be addressed by using two lasers. However, in order to save cost, it would be desirable to vary the power output of a single laser. The laser beam power can be controlled by varying the input power to the laser itself or by jittering the mirrors delivering the beam to the workpiece. Neither of these latter solutions were particularly desirable due to their complexity and the fact that these approaches would slow the operation of the system. Accordingly, it would be desirable to produce two different intensity beams in a manner which is simple and low in cost.
Another problem which is encountered when trying to implement an automated machining system is the fact that due to the composite material used, encapsulated chip packages undergo non-uniform shrinkage during fabrication. It has been found that the extent of the shrinkage problem actually increases as the size of the silicon die (and hence the number of leads on the package) is increased. Because the shrinkage is non-uniform, the exact locations of the leads and dambars for each package can not be predicted. Thus, in order to design an automated device, the laser machining system must be combined with a vision system which can locate the dambars on the package so that they may be treated with the laser beam.
When combining a vision system with a laser machining device, various considerations must be addressed. For example, the machining operation will create smoke and debris which must be removed from the machining site so that it will not obscure the vision system. Moreover, this removal should be in a direction away from the incoming laser beam such that it does not interfere with the delivery optics or reduce the power of the beam delivered to the workpiece. Accordingly, it would be desirable to provide an improved vacuum fixture for holding the integrated circuit chip package in a manner to enhance the operation of the vision system and laser machining operation.
Accordingly, it is an object of the subject invention to provide a new and improved apparatus for processing leads of a semiconductor chip package.
It is another object of the subject invention to provide a new and improved apparatus for processing leads of a semiconductor chip package which includes a combination vision system and laser processor.
It is a further object of the subject invention to provide a new and improved apparatus for the precision automatic removal of dambars from a semiconductor chip package.
It is still a further object of the subject invention to provide an apparatus which can automatically remove both dambars and plastic flash trapped between the leads of a semiconductor chip package.
It is still another object of the subject invention to provide a processing apparatus where the power of the laser beam can be easily controlled.
It is still a further object of the subject invention to provide an apparatus for processing integrated circuit chip packages including a means for attenuating a laser beam to permit both dambar removal and deflashing operations.
It is still another object of the subject invention to provide laser machining apparatus where a defocusing lens is moved into the beam of a laser to change the intensity of the beam that reaches the workpiece.
It is still a further object of the subject invention to provide an improved fixture for use in a laser machining apparatus which includes a means for drawing gas down past the workpiece and away from the laser optics.
It is still another object of the subject invention to provide a fixture for holding a workpiece in a processing apparatus which enhances the image in a machine vision system.